The present invention relates to an initial charge core of a fast breeder and also to a method of charging the core with fuel. More particularly, the invention is concerned with an initial charge core of a fast breeder and a method of charging the reactor core with fuel, which are suitable for suppressing the fluctuation of power distribution in the transition period until the equilibrium core is obtained.
The fast breeder is a nuclear reactor of the type in which fast neutrons produced as a result of fission in the core are absorbed by a fertile material to produce new fissile material in the core. Namely, in the fast breeder, the new fissile material is produced at a rate higher than the rate of consumption of the fissile material by fission. It is possible to make an efficient use of the nuclear fuel by this production of new fissile material, i.e. by the breeding.
The core of the fast breeder usually has a columnar construction consisting of a driver core region containing the fissile material and a blanket region surrounding the driver core region and consisting mainly of the fertile material. The blanket region has a radial blanket region which surrounds the periphery of the driver core region and axial blanket regions which are disposed at both axial ends of the driver core region. The fissile material residing in the driver core region is mainly plutonium 239, while the fertile material in the blanket region consists mainly of uranium 238. The uranium 238 absorbs the fast neutrons produced mainly as a result of the fission of the plutonium 239 and is transformed into plutonium 239.
The driver core region is a region which contains the fuels consisting of enriched fissile material. On the other hand, the blanket region is a region which contains the fuels consisting mainly, at least when initial charging, of fertile material such as depleted uranium, natural uranium or the like material.
The most popular construction of the core generally referred to as "homogeneous core" has the driver core region which is divided into an inner core region and outer core region concentric with each other. In the reactor core having the core region divided into concentric regions as stated above, the degree of enrichment, i.e. the ratio of the amount of fissile material to the sum of the amount of the fissile produce and the amount of the fertile material, is greater in the outer core region than in the inner core region.
A reactor core called Parfait type core, employing a flat columnar inner blanket region having a uniform thickness and disposed at the axially central portion of the driver core region of the fast breeder, has been proposed by G. A. DUCAT et al., in "Evaluation of the Parfait Blanket Concept for Fast Breeder Reactors", C00-2250-5, MITNE-157, Massachusettes Institute of Technology, January 1974.
Also, K. Inoue et al. have proposed a core construction for fast breeders, in which an inner blanket region is disposed at the axial central portion of the driver core region as in the case of the Parfait type core, wherein the axial thickness of the inner blanket region is made greater at the peripheral portion of the inner blanket region than at the central portion of the same ("A Fast Breeder Core With Internal Blanket", Americal Nuclear Society Transactions, Vol. 33, p862 to 863, 1979).
The outer surface of the inner blanket region of this reactor core is in contact with the inner surface of the radial blanket region.